It is known that the crystal grain size of an R2T4B compound (where R is at least one of the rare-earth elements, T is either Fe alone or Fe and Co, and B is boron), which is included as a main phase in an R-T-B based sintered magnet, is one of the factors that determine the performance of the magnet. And it is generally known that the coercivity can be increased by reducing the size of crystal grains in the sintered magnet.
However, if the size of finely pulverized powder particles (i.e., the diameter of the powder particles) were reduced to decrease the size of crystal grains in a sintered magnet, then the overall surface area of the powder particles would increase, and therefore, the amounts of impurities such as oxygen to be adsorbed onto the surface of the particles would also increase. In such a situation, a part of a rare-earth element R included in the material alloy would react to that oxygen and would be consumed to produce an oxide. As a result, the mole fraction of the rare-earth element R (which will be referred to herein as an “R mole fraction”) would be short of the required level. With such an insufficient R mole fraction, a liquid phase (i.e., an R-rich phase) would not be produced smoothly, even though such a liquid phase must be produced to advance the sintering process. To overcome such a problem, however, there is no choice but to increase the R mole fraction in the material alloy too much to avoid a decrease in remanence. That is why even if the pulverized powder particle size were simply decreased, a high-performance magnet could not still be produced.
On top of that, if the overall surface area of a powder compact increased as the finely pulverized powder particle size decreases, then the interfacial energy would increase so much that an abnormal grain growth would arise easily during the sintering process, thus making it difficult to obtain a sintered magnet that has a desired uniform and fine texture. Consequently, high coercivity cannot be achieved just by decreasing the finely pulverized powder particle size.
Patent Document No. 1 shows the relation between the crystal grain size and the performance of a magnet (see FIGS. 3 and 4, in particular), and says that the coercivity increases most significantly at a crystal grain size of around 3 to 5 μm.
Patent Document No. 2 discloses how the coercivity is affected by the addition of various elements, and says that great coercivity can be achieved by adding Mo or Hf when the main phase crystal grains have a size of 5 to 20 μm.
As to the technique of reducing the size of main phase crystal grains of a sintered body, however, both of these patent documents just teach pulverizing the material alloy to a target particle size using a ball mill. To reduce the size of the pulverized powder particles sufficiently by such a known pulverization method, the pulverization process should be carried out either for a long time or a number of times repeatedly with the media changed as needed. In any case, the amounts of impurities will naturally increase and a composition with a high R mole fraction cannot help being adopted. For that reason, the method disclosed in Patent Document No. 1 or 2 cannot be used to make a high-performance magnet.
Patent Document No. 3 discloses that a different phase such as a rare-earth oxide or carbide will check the growth of crystal grains during sintering (i.e., the production of crystal grains with excessively large sizes). However, as such a different phase that will not contribute to improving the magnetic properties must be used according to such a method, the remanence will decrease inevitably, and therefore, it is difficult to apply such a method to making a high-performance magnet.
Patent Document No. 4 discloses a technique for increasing the coercivity without using Tb or Dy by adjusting the crystal grain size of a sintered magnet within a particular range. According to such a technique, however, the increase in crystal grain size is minimized using oxygen that is an impurity. That is why it is also difficult to achieve high remanence, or make a high-performance magnet, by such a technique.
Patent Documents Nos. 5 and 6 disclose a technique for reducing the size of the main phase crystal grains of a sintered magnet by using an additive element such as Nb or Zr, and says that the magnet can be magnetized more easily as a result. According to such a method, the coercivity can be certainly increased sufficiently with the abnormal grain growth minimized during the sintering process. However, as a compound phase that will not contribute to improving the magnetic properties is included in the magnet, the remanence should naturally decrease, and the performance of the magnet can be improved only to a limited degree.
Patent Document No. 7 discloses a technique for reducing the particle size of the powder obtained by a pulverization process with the amounts of oxygen and other impurities minimized and for performing a sintering process at a low temperature without compacting the sintered body using a die. However, Patent Document No. 7 does not mention at all a specific means for pulverizing the powder to the particle size disclosed using a jet mill without increasing the amounts of impurities such as oxygen. Furthermore, Patent Document No. 7 does disclose the amount of oxygen in the finely pulverized powder but does not disclose the composition of the sintered magnet or the amounts of impurities such as oxygen. According to the technique disclosed in Patent Document No. 7, the finely pulverized powder is not compacted with a press machine but is just loaded into a container to a predetermined density and then sintered as it is. That is why to advance the sintering process at a low temperature, a lot of liquid phase components are needed at the sintering temperature. As a result, a lot of rare-earth element R (such as 31.5 mass % of Nd as mentioned in specific examples of Patent Document No. 7) must be used, and such a technique cannot be used to make a high-performance magnet. On top of that, as a lot of liquid phases are generated during the sintering process, the sintering process could be promoted too much to avoid the abnormal grain growth of the sintered texture even if the sintering temperature is lowered.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 59-163802    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 59-211558    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 4-7804    Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2004-303909    Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2005-197533    Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 2006-100847    Patent Document No. 7: Japanese Patent Application Laid-Open Publication No. 2007-180374